In IC technology there is a need to stack chips (dies) together to form multi-layered (3-D) IC devices. One method to form a 3-D device is to bring two (or more) layers together and then encapsulate the layers into a single structure. Electrical conductors and/or contacts on the surfaces of the respective layers serve to carry electrical signals between circuits on the different layers. These conductors/contacts are very small, on the order of microns in diameter, and when exposed to a corrosive atmosphere will corrode relatively quickly. Corrosion then interferes with the signal processing capability of the 3D devices.
The corrosion results from a small gap created between the two dies when they are brought together. Corrosive material, such as water and oxygen can be trapped within these gaps. This trapped corrosive material then interacts with the metallic conductors/contacts resulting in reliability problems.
One solution is to fill the “gap” with filler material. Because the gap size is not constant, the amount at filler is not constant. Thus, it is difficult to completely fill the gap. On the other hand, using too much filler will increase the size of the resultant 3-D device, thereby changing the form factor.
Another solution is to eliminate the gap or make it very small. To accomplish this, the surfaces of the respective dies would have to be extremely planar, thereby adding to the cost of the device, as well as to the cost of handling the dies.
An additional problem is that gases trapped between the layers expand during temperature increases or external pressure decreases. The expanded gases exert separation pressure on the bonded tier.